1. Field of the Invention
The present invention relates generally to apoptosis or programmed cell death, and more particularly, to amino acid sequences of proteins that are associated with apoptosis and the use of these proteins in the diagnosis, prevention and treatment of disorders associated with abnormal cell proliferation and apoptosis.
2. Background of the Related Art
Programmed cell death or apoptosis plays a critical role during animal development by functioning in the destruction of unneeded cells and tissues. The term programmed death was established to distinguish physiological or genetic-regulated cell death from necrotic cell destruction. Genetically regulated cell death is an integral component of normal development, and is used in processes such as, limb formation and nervous system remodeling. Cell death is also involved in removal of abnormal cells during development, including those during tumor genesis.
It has now become clear that disturbances in programmed cell death, which prevent or delay normal cell turnover, can be just as important to the pathogenesis of diseases as known abnormalities in the regulation of proliferation and the cell cycle. Like cell division, which is controlled through complex interactions between cell cycle regulatory proteins, programmed cell death is similarly regulated under normal circumstances by the interaction of gene products that either induce or inhibit cell death.
The stimuli that regulate the function of these apoptosis gene products include both extracellular and intracellular signals. Either the presence or the removal of a particular stimulus can be sufficient to evoke a positive or negative apoptosis signal. For example, physiological stimuli that prevent or inhibit apoptosis include growth factors, extracellular matrix, CD40 ligand, viral gene products, neutral amino acids, zinc, estrogen and androgens. In contrast, stimuli that promote apoptosis include growth factors such as tumor necrosis factor (TNF), Fas and transforming growth factor β (TGF β). Other stimuli that promote apoptosis include, for example, neurotransmitters, growth factor withdrawal, loss of extracellular matrix attachment, intracellular calcium and glucocorticoids. Although apoptosis is mediated by diverse signals and complex interactions of cellular gene products, the results of these interactions ultimately feed into a cell death pathway that is evolutionarily conserved between humans and invertebrates.
Several gene products that modulate the apoptotic process have now been identified. Although these products can be generally separated into two basic categories, gene products from each category can function to either inhibit or induce programmed cell death. One family of gene products is related to the protein Bcl-2, which inhibits apoptosis when overexpressed in cells. A second family of gene products, the aspartate-specific cysteine proteases (ASCPs), are genetically related to the ced-3 gene product, which was initially shown to be required for programmed cell death in the roundworm, C. elegans (Ellis et al., 1991)
A series of elegant genetic studies of programmed cell death in the worm C. elegans led to the isolation of the ced-3, ced-4 and ced-9 genes. Ced-3 is homologous to the mammalian family of caspases, which upon proteolytic activation are critical effectors of the programmed cell death signaling pathway (Cryns and Yuan, 1998). CED-4 is homologous to mammalian Apaf-1, which activates caspases in the presence of cytochrome c and dATP. CED-9 is a member of the Bcl-2 family of cell death regulators and found to represses the apoptosis of many different cells in the nematode development.
Apoptosis maintains tissue homeostasis in a range of physiological processes such as embryonic development, immune cell regulation and normal cellular turnover. Therefore, the dysfunction or loss of regulated apoptosis can lead to a variety of pathological disease states. Clearly, there is a need for factors that are useful for inducing apoptosis for therapeutic purposes, for example, as an antiviral agent, an anti-tumor agent to control embryonic development and tissue homeostasis, and the roles of such factors in dysfunction and disease. Further, there is a clear need for factors that are useful for reducing or halting apoptosis for therapeutic purposes, for example, to treat diseases caused or associated with apoptosis, such as Alzheimer's disease, Parkinson's disease, rheumatoid arthritis, chronic inflammation, acute inflammation, AIDs, degenerative liver disease and the like. Treatments that are specifically designed to modulate the apoptotic pathways in these and other pathological conditions can change the natural progression of many of these diseases.
Genetic screens in Drosophilia have revealed three cell death inducer genes: reaper (rpr) head involution defective (hid) and grim. The function of these genes is required for virtually all programmed cell death during embryogenesis. Ectopic expression of each of these genes is also sufficient to induce caspase-dependent programmed cell death in a wide range of cell types. Although vertebrates homologs of rpr, hid and grim have not yet been identified, expression of each of these genes is sufficient to induce apoptosis in mammalian cells (McCarthy and Dixit, 1998; Haining et al., 1999) suggesting that the downstream death pathway may be conserved between flies and mammals.
The similarity between vertebrate and invertebrate cell death pathways establishes Drosophila as a valuable model system for genetics studies of the regulation of apoptosis in humans and thus could be the vehicle to identify new apoptotic genes and their gene products to modulate apoptosis for the therapeutic treatment of human diseases.